Stephen Millanta wants the justice system to show no mercy to the killer who punched and then stomped on his son, damaging his liver and causing massive internal bleeding.

Police found Izak Wikaira Millanta, 17, lying unresponsive in a car park at Coastlands mall, Paraparaumu, early on August 11.

Paramedics fought to save him for nearly an hour before he was taken to Wellington Hospital for emergency surgery. He died in the early hours of August 12.

There was a heavy police presence at the Porirua District Court yesterday as Manawanui Stewart, 25, pleaded guilty to his manslaughter.

Stewart also admitted assaulting Izak's older brother, Tane, on the night of the bashing.

He was remanded in custody to appear in the High Court at Wellington for sentencing in February.

After the appearance, Stephen Millanta told The Dominion Post he would never forgive his son's killer. Stewart showed no mercy to Izak and he hoped the justice system would do that to Stewart.

"Whatever the maximum [sentence], that's what I want him to do. I don't want any remorse shown at all."

Earlier this year, Izak's parents said they believed there was a second man who killed their son. They appealed for anyone hiding information to have a conscience and speak to police.

Mr Millanta said he still believed Stewart was "taking the rap" for an attack carried out by two men. "I feel sorry for him because he's going to do some time. He's going to pay."

Police said they would consider any new information that came to light.

Christine Wikaira said she was relieved someone had been held accountable for her son's death. However, the family would continue to experience "overwhelming sadness" for the rest of their lives.

The police summary of facts said Stewart had been drinking on the night of the attack and was involved in a series of scuffles outside the Retro Bar. Izak and his friend Jahze Wawatai arrived during one of the fights.

Mr Wawatai was allegedly assaulted and Stewart told Izak to leave. Stewart told police he took exception to Izak laughing and being cheeky as he backed away down an alleyway, which led to the Coastlands car park.

He and his friend chased Izak, who yelled for them to "leave him alone and that he was sorry for being smart to him", the police summary said.

Stewart caught up to him after about 300 metres and pushed him to the ground. He then pinned him down and punched him twice in the face before his friend told him to let him go.

Stewart got off Izak and took two step backwards before stomping on his abdomen. He kept his foot down and applied pressure, digging his heel in and leaning forward to put a substantial amount of his 100kg body weight on Izak's abdomen.

He then leaned over Izak and told him "he deserved what he got" because of the way he had acted, the summary said.

"Seeing the victim clutching his abdomen in pain and moaning, the defendant states he rolled the victim on to his side, effectively putting him in the recovery position."

As Stewart walked away, he looked back to see the victim roll from his side on to his back still clutching his abdomen and moaning.

He and his friend then avoided a police patrol that had arrived in the car park and went to a friend's house to continue drinking.

An autopsy showed Izak died as a result of blunt impact trauma to his torso and head, with a cut liver, internal bleeding and a broken nose.

Stewart told police he wanted to "teach him a lesson" but had no idea his actions would lead to Izak's death.